Noise filtering is a very important operation in digital image processing. For example, the pixels in a signal produced by an image sensor such as a CCD (charge-coupled device) or CMOS (complementary metal-oxide-semiconductor) device may be corrupted by several types of noise, such as reset (or kTC) noise, photon shot noise, dark current, and thermal noise. The characteristics of this noise may be related to factors such as the particular fabrication process, local variations in the sensor or other circuitry such as amplifiers, and ambient conditions (e.g. temperature, level of illumination). The noise may include components that are characterizable, such as, but not limited to multiplicative noise, and/or components that are random.
During processing of the image signal, the noise may be amplified, and the visual quality of the final image may be reduced. Noise problems may be especially acute in particular situations, such as mobile imaging applications that use compact sensor modules having small pixel areas (e.g. in cellular telephones, personal digital assistants (PDAs), or wearable devices such as wristwatches). Moreover, these situations may demand solutions that are computationally inexpensive.
Noise problems may also arise in imaging applications that make use of nonvisible radiation. For example, high-frequency or impulsive noise may also be manifested as speckle in synthetic aperture radar (SAR) images, which are typically generated using microwave radiation.